1. Field of the Invention
The present invention relates to a lens device having a structure for preventing a lens element from being detached from a lens holding flame.
2. Description of Related Art
Lens devices such as a wide angle lens and a telephoto lens are used for various optical instruments including cameras. Such a lens device comprises a lens barrel and a lens system having a plurality of lens elements which are held in the lens barrel by various lens holding mechanisms. A typical lens holding mechanism includes a cylindrical or annular lens holding fixture frame in which a lens element is bonded by an adhesive and fixedly held. The lens holding fixture frame is moved in an axial direction in the lens barrel for zooming or focusing by means of a cam mechanism. When fixing the lens element into the lens holding fixture frame, it is usual to use jigs and fixtures for position adjustment of the lens element with respect to the lens holding fixture frame. In order to allow the lens element to be adjusted in position, there is provided an annular clearance between the lens element and the lens holding fixture frame. After position adjustment, an adhesive is filled in the annular clearance to fix the lens element to the lens holding fixture frame.
Before describing the present invention in detail, reference is made to FIGS. 6 and 7 for the purpose of providing a brief background that will enhance an understanding of the feature of the present invention. FIG. 6 shows a prior art lens device 1′ having a lens holding structure for holding a plastic lens element in a diametrical cross section. As shown, the lens device 1′ has an annular lens holding fixture frame 2′, a plastic lens element 3′ fixedly held in the lens holding fixture frame 2′ and a lens barrel 4 in which the lens holding fixture frame 2′ is movably received. The lens holding fixture frame 2′ is moved along an axis L of a lens system including the lens element 3′ in the lens barrel 4 by means of a cam mechanism which is not shown and, however, known in various forms in the art. The lens holding fixture frame 2′ is provided with a rear frame portion 11′. This rear frame portion 11′, which is formed as an integral part of the lens holding fixture frame 2′, has a plurality of holes 9′ circumferentially arranged at regular angular intervals which are used to adjust the lens element 3′ in position. After completion of the adjustment, an adhesive is applied into an annular clearance between the lens element 3′ and the lens holding fixture frame 2′ through some of the holes 9′ so as to fix the lens element 3′ to the lens holding fixture frame 2′. The lens holding fixture frame 2′ is further provided with an inner annular flange 10′ serving as a positioning wall against which a flange portion 8′ of the lens element 3′ is pressed so as thereby to be axially positioned. On the other hand, the lens element 3′ has a lateral surface 7′ tapering from a front edge thereof so that an annular clearance is formed between the lens element 3′ and the lens holding fixture frame 2′. An adhesive 12, which is filled partly in the annular clearance through some of the holes 9′, cures to fix the lens element 3′ in the rear frame portion 11′ of the lens holding fixture frame 2′.
The lens element 3′ is injection molded by the use of a mold 13′ such as shown in FIG. 7. The mold 13′ comprises an upper mold half 13U and a lower mold half 13L between which a mold cavity 14′ is formed. The lower mold half 13L has a flank 14a′ forming a peripheral wall of the mold cavity 14′ which has a predetermined draft angle and is provided with a plurality of ejection pins 15′ arranged correspondingly to an annular shoulder 14b′ for forming a front base surface of the flange portion 8′ of the lens element 3′. The lateral surface 7′ of the lens element 3′ tapers backwards at an angle which is derived from the draft angle of the flank 14a′ of the mold cavity 14′.
A problem encountered by the lens device 1′ is that the lens element 3′ causes displacement with respect to or falls out of the lens holding fixture frame 2′ due to adhesive transfer which arises from thermal expansion of the lens holding fixture frame 2′ or an external impact on the lens holding fixture frame 2′. If the lens element 3′ causes displacement in the lens holding fixture frame 2′, the lens device 1′ looses its given optical characteristics or ceases to function properly and, as a consequence, disables the optical instrument itself for use. One of means for eliminating the problem which is disclosed in, for example, Japanese Unexamined Patent Publication No. 11-211957 is to form a plurality of V-shaped grooves in an interface between a lens element and a lens holding fixture frame. An adhesive feeds into the grooves and is cured therein so as thereby to wedge up the lens element in the lens holding fixture frame.
However, it counts against requirements for a simple shape of the lens holding fixture frame and a high injection molding yield of lens element to form the V-shaped grooves in the lens holding fixture frame.